Metal oxide materials are used for a variety of applications, including catalysis, coatings, sensors, electronics, and semiconductor devices. The physical and chemical properties of metal oxides can be modified by adding one or more other elements to the metal and the oxygen in the material. These added elements are referred to as dopants, and the resulting material is referred to as a doped metal oxide.
For example, tin oxide (empirical formula SnO2) is present in a variety of commercial products. Sensors for gases such as oxygen, hydrocarbons or carbon monoxide may include tin oxide on a solid support. Sensors containing tin oxide typically are heated to 400° C. to enable the sensing function. Heavy doping of the tin oxide may increase its carrier concentration, allowing it to be used for sensing at lower temperatures. Indium tin oxide (ITO), which includes tin oxide as a dopant in the indium oxide, is widely used as a transparent electrical conductor for applications in displays, electro-optic sensors, and light emitting diodes (LEDs). Doping of tin oxide may provide an alternative to ITO, which faces future supply problems due to the scarcity of indium.
In another example, titanium oxide (empirical formula TiO2) has been extensively studied as an efficient photocatalyst under ultraviolet (UV) light. Photocatalysts provide for catalysis of chemical reactions when irradiated by electromagnetic radiation, and have been investigated for applications in a variety of areas, including environmental remediation. Stable catalysts that promote oxidation reactions can break down pollutants in air and/or in water into substances that are less harmful. Anionic doping of titanium oxide can produce a red-shift in the light absorbance of the metal oxide. This can lead to photocatalysis under visible-light irradiation, allowing a greater portion of the solar spectrum to be used for catalytic activity. Among various anion-doped titanium oxides, nitrogen-doped titanium oxide (empirical formula TiO2-xNx) has been explored extensively, due to the ease with which it can be formed.
The chemical and/or physical properties of a metal oxide can be affected by the physical form of the metal oxide. Some metal oxides, such as tin oxide, are amorphous until heated above a transition temperature. Other metal oxides have more than one possible crystal structure, where each crystal structure tends to form at a different temperature range. Some of these metal oxides, such as titanium oxide, have more desirable physical and/or chemical properties when at least a portion of the metal oxide is in one crystal structure than when that crystal structure is absent.
In the example of titanium oxide, the anatase crystal phase is stable at low temperatures, and the rutile crystal phase is stable at high temperatures. Heating titanium oxide above the transition temperature of 700° C. can transform some or all of the anatase phase into rutile phase. Titanium oxide that includes a mixture of the anatase and rutile phases has photocatalytic properties that are superior to titanium oxide that is either pure anatase or pure rutile. The commercially available P25 titanium oxide powder (DEGUSSA) is an example of this dual-phase crystal titanium oxide.
It has been difficult, however, to combine the advantages of doping of metal oxides with the advantages associated with desirable crystal structures. This difficulty is due at least in part to the distinct processing requirements for each of these properties, which are typically mutually exclusive. Anion dopants in a metal oxide tend to become unstable at high temperatures. Thus, doped metal oxides that are exposed to high temperatures typically lose a large proportion of their dopants. In contrast, sometimes desirable crystal structures can only be obtained at high temperatures. Thus, the temperatures at which metal oxides are doped typically cause the formation of lower temperature crystal structures or amorphous materials.
In the example of titanium oxide, dual-phase titanium oxide typically contains little or no nitrogen dopant. Attempts to introduce a nitrogen dopant have so far achieved only a 2% doping concentration (Fu, H. et al., J. Phys. Chem. B, 110 (7), pp. 3061-3065 (2006)). Surface nitriding treatment of other solids, such as iron, is believed to yield a significant nitrogen concentration gradient from the surface to the interior, and nitride phases often form on the surface. Since the nitride of titanium is an electronic conductor, it produces no photocatalytic effect, and its presence on the catalyst surface is to be avoided.
Sol-gel synthesis of titanium oxide, including doped titanium oxide typically requires heating to temperature of 500° C. or greater in order to obtain catalytically active crystalline materials. These temperatures are incompatible with a number of desirable substrates for thin films of these materials, such low-melting point glass and plastics. Even when the film is formed by a low temperature technique, such as by reactive sputtering, heating at high temperatures is still required to form catalytically active material.